Add an EPSG:3857 map to Tissot example

This adds an EPSG:3857 map to the Tissot example. It's interesting to note that circles on the sphere are circles on a Web Mercator map, which is an accordance with what's in the Spherical Mercator Wikipedia page: http://en.wikipedia.org/wiki/Mercator_projection.

![tissot](https://cloud.githubusercontent.com/assets/76594/4274451/322c8b36-3cf4-11e4-930f-1d62fe41aa28.png)